The amount of water we’re talking about is insignificant given the already large reservar of water we have on earth. Also in the absence of other greenhouse gases, the water cycle is self regulating, it doesn’t act as a greenhouse gas in the absence of another sauce of warming (which then increases the moisture carrying capacity of the air, starting a feedback loop).
CO2 is much more rare so adding that same amount in CO2 is very significant.
It’s a bit like making a sauce. Adding a bit more of your main ingredient (for example cream in a cream based sauce) won’t make much difference to how it tastes. But add the same amount of a potent spice and your sauce is ruined.